Touch panel displays are widely used in consumer electronics, such as smartphones and computing tablets, among other devices. Broadly speaking, there are two types of touch panel technologies currently used in consumer electronics: projected capacitance and resistive. Both types of touch panels typically can only sense the location and time of a touch event on the touch panel (e.g., from a finger or stylus). The location of a touch event is typically recorded only in two dimensions (e.g., x-y coordinates). Conventional touch panels are unable to sense in a third dimension to determine the magnitude of a touch force (e.g., a z-coordinate). Prior attempts at three-dimensional sensing have typically focused on the inclusion of a sensitive analog element. Conventionally, the inclusion of an analog element in what is otherwise a digital system has been costly, bulky and non-trivial.
In contrast, in concert with trends in the smartphone industry, and the computing industry more generally, touch panel displays continue to become thinner and less costly. One approach to reduce costs while making displays thinner is to integrate touch panel elements with display elements in a so-called “in-cell” fashion, as opposed to “on-cell” approaches.
On-cell approaches typically provide a touch panel display by stacking transparent touch location sensing elements (e.g., traces for capacitive touch sensing) on top of display elements. In-cell approaches typically provide a touch panel display by interspersing touch location sensing elements between layers of the display elements. For example, in one approach, touch location sensing traces may be provided between the liquid crystal layer and a color filter layer in an LCD device.